Have you tried a old coil yet ?. I would recommend a Lucas one from the 1960s.
It cured the problem you have for me..
Martin

Location: Chichester

Not yet Martin, I am trying to isolate the problem by doing one suggested remedy at a time.

Location: East of Sandy

As part of routine maintenance I notice the CB points gap is mighty wide - around 20 thou. What effect would this have on the coil's performance?

Location: East of Sandy

That would reduce the dwell angle, i.e. the points would open sooner and close later so the coil would have a shorter duty cycle and therefore run cooler.

Ian Mc.

Location: Shropshire

Bu**er, it wouldn't be that then. May put an old coil on at the weekend and see if that helps - snow permitting!

Location: East of Sandy

Hello Brian,
I'm pleased you might try a old coil. For the first 35 years working as a mechanic I never replaced a coil ( they just did not fail ) however in the last 5 years I have replaced 7, the last one was on a morris 1000 it is positioned on top of the dynamo next to the engine block therefore it overheats even quicker.
Martin

Location: Chichester

One factor could be plug gaps; Austin always recommended 0.018" to 0.020" for sevens in period but I wouldn't be surprised if today most people set them to 0.025" or even greater.

Ian Mc.

Location: Shropshire